It is a principle of thermodynamics that the greater the difference in temperature at an interface the higher the efficiency of heat transfer through the interface. This principle, as applied to forced air heating systems, indicates that the heat exchanger should be operated at as low a temperature as will allow sufficient hot air to be produced.
Many forced air heating systems, particularly older ones, operate the heat exchanger at an undesireable excessively high temperature. The result is a loss of furnace efficiency, or from another point of view an unnecessary waste of energy up the flue. In addition, an excessively hot exchanger produces excessively hot heating air which most often results in household temperature overshoot, and consequently undersireably large swings in ambient air temperature.
As summarized and described in detail below, the present invention provides an "add-on" or auxiliary furnace controller which corrects or mitigates heat exchanger overheating. The invention may thus be installed in a heating system, particularly an older one, to improve its efficiency, and to improve household temperature stability.